The present invention relates to a variable capacity vane pump used in a pressure fluid utilizing equipment, e.g., a power steering system that decreases the steering wheel control force of an automobile and, more particularly, to a variable capacity pump which, in a loaded state accompanying the operation of a pressure fluid utilizing equipment, performs flow rate control in response to the load pressure.
Generally, a capacity vane pump directly driven and rotated by an automobile engine is used as a pump for a power steering system. In this capacity pump, the discharge flow rate increases or decreases in accordance with the engine speed. Thus, this capacity pump has characteristics opposite to the auxiliary steering force of the power steering system which increases while the automobile stops or travels at a low speed and decreases while the automobile travels at a high speed. A capacity pump having a large capacity must be used that can assure a discharge flow rate enough to obtain a necessary auxiliary steering force even in low-speed travel with a low engine speed. For high-speed travel at a high engine speed, a flow control valve for controlling the discharge flow rate to a predetermined amount or less is indispensable. For these reasons, in the capacity pump, the number of components is increased and the structure and the arrangement of the passages are complicated, inevitably leading to an increase in size and cost of the pump as a whole.
In order to solve these inconveniences of the capacity pump, many variable capacity vane pumps capable of decreasing the discharge flow rate per revolution (cc/rev) in proportion to an increase in engine speed are proposed in, e.g., Japanese Patent Laid-Open Nos. 53-130505, 56-143383, and 58-93978, Japanese Utility Model Publication No. 63-14078, and Japanese Patent Laid-Open No. 7-243385. In these variable capacity pumps, a flow control valve as that needed in the capacity vane pump is not required, the waste in drive horsepower is prevented to improve the energy efficiency, a return flow to the tank is absent to prevent an oil temperature increase, and problems such as leakage in the pump and a decrease in volumetric efficiency can be prevented.
An example of such a variable capacity vane pump will be briefly described with reference to FIG. 8 showing the pump structure of Japanese Patent Laid-Open No. 7-243385. Reference numeral 1 denotes a pump body; 1a, an adapter ring; and 2, a cam ring. The cam ring 2 can swing and displace in an elliptic space portion 1b formed in the adapter ring 1a of the pump body 1 through a support shaft portion 2a as the swing center. A biasing force is applied to the cam ring 2 by a coil spring serving as a press means (not shown) in the direction of an arrow F.
A rotor 3 is housed in the cam ring 2 to be eccentric to one side such that it forms a pump chamber 4 on the other side. When the rotor 3 is driven and rotated by an external drive force, vanes 3a held to be radially movable back and forth are moved back and forth. A drive shaft 3b drives the rotor 3 to rotate in the direction of an arrow.
Reference numerals 5 and 6 denote a pair of high- and low-pressure fluid pressure chambers formed on the two sides of the outer circumferential portion of the cam ring 2 in the elliptic space portion 1b of the adapter ring 1a of the pump body 1. Passages 5a and 6a respectively open to the fluid pressure chambers 5 and 6 through a spool type control valve 10 (to be described later). The passages 5a and 6a guide the input and output fluid pressures of a variable orifice 12 formed in a pump discharge passage 11 as control pressures for swinging and displacing the cam ring 2. When the input and output fluid pressures of the variable orifice 12 of the pump discharge passage 11 are introduced to the cam ring 2 through these passages 5a and 6a, the cam ring 2 is swung and displaced in a required direction to change the volume in the pump chamber 4, thereby controlling the discharge flow rate in accordance with the pump discharge flow rate. In other words, the discharge flow rate control is performed such that the discharge flow rate is decreased with an increase in pump speed.
A pump suction opening (suction port) 7 opens to a pump suction region 4A of the pump chamber 4, and a pump discharge opening (discharge port) 8 opens to a pump discharge region 4B of the pump chamber 4. These openings 7 and 8 are formed in either one of a pressure plate and a side plate (neither are shown) that serve as fixing wall portions for holding pump constituent elements comprising the rotor 3 and the cam ring 2 by sandwiching them from the two sides.
The biasing force is applied to the cam ring 2 by a coil spring from the fluid pressure chamber 6, as indicated by F in FIG. 8, to normally maintain the volume in the pump chamber 4 to the maximum. Seal members 2b are formed in the outer circumferential portion of the cam ring 2 to separately form the fluid pressure chambers 5 and 6 on their right and left sides together with the support shaft portion 2a.
Hair-like notches 7a and 8a are formed continuous to the terminal end portions of the pump rotational directions of the pump suction opening 7 and the pump discharge opening 8, respectively. When the distal ends of the vanes 3a come into slidable contact with the inner circumferential portion of the cam ring 2 during rotation of the rotor 3 to perform a pumping operation, the notches 7a and 8a gradually relieve the fluid pressure from the high pressure side to the low pressure side between a space formed between the two vanes 3a close to the end portions of the openings 7 and 8 and a space formed between the two vanes 3a adjacent to the vanes 3a described above, so that a surge pressure and pulsation caused by the surge pressure are decreased.
The spool type control valve 10 is actuated by a difference pressure between the input pressure and the output pressure of the variable metering orifice 12 formed midway along the pump discharge passage 11. A fluid pressure corresponding to the pump discharge flow rate is introduced from the control valve 10 to the high-pressure fluid pressure chamber 5 outside the cam ring 2, to maintain a sufficient flow rate at the initial stage of pumping operation. Especially during a loaded state caused by the operation of the pressure fluid utilizing equipment, when the difference pressure between the input pressure and the output pressure of the variable orifice 12 becomes equal to or higher than a predetermined value, this control valve 10 introduces the output fluid pressure of the variable orifice 12 as a control pressure to the high-pressure fluid pressure chamber 5 outside the cam ring 2. Thus, even when unbalanced forces act due to the non-equilibrium fluid pressures inside and outside the cam ring 2, e.g., even when a force that decreases the discharge flow rate from the pump acts, this acting force can be canceled, thereby preventing swing of the cam ring 2.
In other words, in this spool type control valve 10, the pump suction opening 7 and the pump discharge opening 8, that open to the pump chamber 4 on the two sides of the swing direction as the support shaft portion 2a serving as the swing fulcrum of the cam ring 2 as the center, are arranged in an unbalanced state in the structure of the variable capacity pump described above. A control operation is performed so that the cam ring 2 will not be swung about the support shaft portion 2a as the center by the right and left unbalanced forces generated due to the position of the pump discharge opening 8.
This will be described in detail. In the pump cartridge (pump actuating portion) having the pump constituent elements, e.g., the rotor 3, the cam ring 2, and the like, of the vane pump as described above, small chambers (chambers partitioned by two vanes 3a) located in intermediate regions (portions in FIG. 8 where the openings 7 and 8 do not exist) corresponding to a region extending from the end point of the suction region 4A to the start point of the pump discharge region 4B of the pump chamber 4 and a region extending from the end point of the pump discharge region 4B to the start point of the suction region 4A change alternately to the pump discharge pressure and the pump suction pressure.
When a vane 3a preceding in the rotating direction of the rotor 3 reaches the opening 8 or 7 at the leading end in the rotational direction, a small chamber formed by this vane 3a and a following vane 3a is set at the pump discharge or suction port pressure of the corresponding one of the openings 7 and 8. When this following vane 3a is located at the opening 7 or 8 at the trailing end in the rotational direction, the small chamber is set at the pump discharge or suction port pressure of the opening 8 or 7.
Accordingly, in this variable capacity vane pump, the position where the small chamber set at the high-pressure pump discharge pressure corresponds to two regions indicated by .theta.1 and .theta.2 (.theta.1&lt;.theta.2) on the left and right sides of a line segment extending through the centers of the support shaft portion 2a and drive shaft 3b in FIG. 8. In these pump discharge region portions, the right and left portions about the line segment extending through the support shaft portion 2a described above as the center become unbalanced. In particular, when such unbalanced forces act on the cam ring 2, the higher the pump discharge pressure, the more inconveniences appear.
The pump discharge pressure increases in a loaded state, e.g., during steering wherein a power steering system PS serving as a pressure fluid utilizing equipment operates, or when the pump speed increases even if no load acts. In such a loaded state and the like, when the pump discharge pressure increases, the cam ring 2 swings in a direction to reduce the pump chamber 4 (to the right in FIG. 8) due to the pressure difference among the internal pressure in the cam ring 2 and the pressures in the outer fluid pressure chambers 5 and 6. When the cam ring 2 swings in this manner, the pump chamber 4 reduces during the loaded state that requires a discharge flow rate, so that the discharge pressure and the discharge flow rate decrease.
The spool type control valve 10 is formed to decrease a fluid pressure P3 of the fluid introduced to the high-pressure fluid pressure chamber 5 outside the cam ring 2, to be lower than a pressure P1 on the upstream of the variable orifice 12 in the pump discharge passage 11, so that the cam ring 2 will not be swung or displaced even by the unbalanced forces (described above) during the loaded state of the cam ring 2. A pressure P2 on the downstream of the variable orifice 12 is introduced to the low-pressure fluid pressure chamber 6 outside the cam ring 2. This pressure P2 is lower than the pressure P1 described above and higher than the pressure P3.
According to the variable capacity vane pump having the arrangement described above, since unnecessary swing and displacement of the cam ring 2 occurring during the loaded state are prevented in accordance with fluid pressure control by providing the control valve 10, the structure of the entire pump becomes complicated. When this control valve 10 is used, the leakage amount of the fluid in the pump becomes large.
In this conventional structure, even in the unloaded state, the larger the pump speed, the higher the discharge pressure. Thus, the control valve 10 performs flow rate control similar to that in the loaded state described above. In the unloaded state, however, the flow rate is excessively large, and the pump drive torque also increases. For example, in a variable capacity vane pump applied to a power steering system, the effect of decreasing the load to the engine is small, and accordingly the fuel consumption cannot be improved much better than in the conventional system.
In particular, when a variable capacity pump of this type is compared with a capacity vane pump, one of the purposes is to eliminate a valve that performs flow rate control. To use the spool type control valve 10 as described above for controlling the fluid pressures to be supplied to the fluid pressure chambers 5 and 6 outside the cam ring 2 is against this purpose. Therefore, strong demand has arisen for solving the inconveniences caused by the unbalanced forces acting on the cam ring during the loaded state as described above by using a different scheme.